The present invention relates to a variable speed controllable induction motor by which speed control over a wide range can be easily effected and in which the torque characteristics and efficiency are excellent.
A squirrel-cage rotor type induction motor is widely used as a prime mover as it is simple in its construction, less costly, less susceptible to damage and, moreover, both the power factor and efficiency are good in operation of the motor. However, drawbacks of the squirrel-cage rotor type induction motor are that the starting characteristics thereof are not good, that is, a starting torque is not sufficiently satisfactory despite high current consumption at starting and that it is difficult to effect a wide range speed control with high efficiency.
In a wound rotor type induction motor in which a secondary resistor is inserted externally through brushes and slip-rings in the circuit of rotor windings, a continuous and efficient speed control can be effected comparatively simply by means of controlling the resistance values to thereby change the slip of the motor. However, in the squirrel-cage rotor type induction motor, the structure or configuration thereof does not permit such a method of control and, therefore, the methods generally adopted are as explained hereunder.
As methods of continuous control of speeds, one method is by way of varying a power source frequency and another is by changing a power source voltage. Both of these methods are generally not satisfactory and have drawbacks in that efficiency of the control is low at low speed ranges. Further, the former method requires a high cost frequency convertor which, in the course of converting frequencies, produces high harmonic noise and other electromagnetic waves which, if they flow into the commercial power lines, would cause the occurrence of various harmonic noise problems, causing malfunctioning of computers or other electronic devices, heating of power capacitors, etc.
Further, as a method of continuous control of speeds, it is known to arrange that two induction motor components of the same poles are axially disposed side by side and two stators having primary windings cause, by either mechanical or electrical means, a phase difference between the rotating magnetic fields generated around each of the two rotor cores of a single rotor provided on a rotary axis so that a speed control is effected by varying the phase difference thereby to change the amount of the combined secondary voltage, that is, to change the amount of the secondary current flowing in the conductive members of the rotor.
Also, there has been known a system in which several kinds of poles are provided in stator winding circuits of the motor and a speed control is performed by means of switching the poles during the operation. While this system enables a speed to be controlled step-wise or noncontinuously, it is not possible to perform a stepless and smooth control of the speed by this system.
The applicant of the present application filed a U.S. patent application, U.S. Ser. No. 055,147, on May 28, 1987 now U.S. Pat. Ser. No. 4,785,213. The invention covered therein relates to a variable speed controlled induction motor characterized by the combination of:
a rotor formed in one-piece having a plurality of rotor cores mounted, with a predetermined air space or a non-magnetic core portion being provided therebetween, on a common axis and having a plurality of conductive members interconnected and respectively mounted on the rotor cores;
a plurality of stators having a plurality of stator cores disposed side by side and surroundingly facing the respective rotor cores and having stator windings wound respectively on the stator cores, the stator windings being connected or coupled in series with respect to the power source;
connecting members short-circuiting the conductive members at the air space or non-magnetic core portion disposed between the rotor cores; and
phase shifting means for producing phase differences between the voltages induced on the portions of the conductive members which face one of the plurality of stators and the voltages induced on the corresponding portions of the conductive members which face another one of the stators.
That invention provides a variable speed controlled induction motor which, as compared with conventional induction motors, is far superior in starting characteristics and in capability of performing an efficient speed control over a wide range. However, since the disclosure for that invention was silent on the character of materials of conductive members so that, if all of the several sets of rotor conductive members were of materials which have the same electric characteristics of conductivity with respect to one another, there was a possibility for the operational characteristics to be limited depending on the selection of such materials because, if all of the conductive members happened to be of materials either all higher resistivity or all lower resistivity, this would of course significantly affect the operational characteristics of the motor. If the conductive members of the rotor are made of higher resistivity materials, it may be possible to obtain a sufficiently high starting torque but the maximum rotation speed at a rating load tends to be somewhat lower as compared with a normal one having the same rating output. On the other hand, if the conductive members of the rotor are made of lower resistivity materials, torque characteristics during starting of the motor and low speed operation of the motor do not show any marked improvement over the conventional ones.